In this specification, the method and circuit described will be related to rear view mirrors for motor vehicles of the type in which the orientation of the mirror glass is adjustable relative to the mirror housing by way of two screw jack drives arranged to adjust the orientation of the mirror glass about mutually orthogonal axes.
However, it would be possible for the circuit elements and method disclosed herein to be used with any moveable mechanism which can be sensed and then controlled to move to predetermined positions.
One example of a sensor system representing linear displacement is disclosed in European Patent 0907068 entitled “Hall Effect Sensor” in the name of Britax Rainsfords Pty Ltd, the applicant in this matter, that is hereby included in this specification by reference.
Furthermore European Patent 0549173 entitled “Rear View Mirror Assembly for Motor Vehicle” also in the name of Britax Rainsiords Pty Ltd and is referred to in European Patent 0907068, that is also hereby included in this specification by reference.
European Patent 0549173 discloses a rear view mirror assembly for a motor vehicle of the type in which a reflective member is mounted for angular movement on a pivotal mounting attached to a mirror housing, with the drive housing located in the mirror housing having a projecting jacking screw which is pivotally coupled to the rear surface of the reflective mirror at a location remote from the mirror's pivotal mounting.
A problem with any position sensing and control arrangement, and in particular for vehicular environments, is the variability of the supply of power to the drive circuit as well as the position sensors of the arrangement. Power supply variability and the nature of the contact between electrical circuits within a vehicle environment can result in sensing control inaccuracies and potential damage to the control circuitry. Furthermore when voltage sensing from a potentiometer (variable voltage divider) is used to determine relative position sensing such as linear or rotational displacement, accuracy can be detrimentally affected as a result of voltage variation, mechanical wear and environmental influences.
Voltage variation can occur when the vehicle experiences hot and cold environments, in addition there is the further complicating factor of voltage noise spikes caused by radio frequency interference (RFI). All these factors can greatly affect the accuracy and hence repeatability of positional displacement because the voltage measurements used are not reliable.
Furthermore, voltage based systems are designed to have “wetting currents” to overcome the expected high resistance joints in cabling and in doing so also minimise transient voltage drops within the circuits caused by contact resistance in the cabling joints. The existence of “wetting currents” further compromise the use of voltage measurements to determine positional displacement.
It is an aim of this invention to minimise or eliminate these types of problems and design compromises.